1. Field of the Invention
This invention relates in general to a semiconductor device and especially to a transistor having a hetero junction.
2. Description of Prior Art
Transistors having a wide gap emitter are known in which the band gap energy width of the emitter is larger than that of the base and there is a hetero junction between an emitter-base junction. In such transistors the minority carrier current in the emitter can be sufficiently small even if the impurity concentration in the base is greater than 10.sup.19 atoms/cm.sup.3 order of magnitude because of the band gap difference at the emitter-base junction. This results in the emitter junction efficiency .gamma., approaching unity and the current gain (emitter-grounded) h.sub.FE is large.
Materials for forming a hetero junction must be such that the differences of lattice constant and the thermal expansion coefficient are very small. The combinations of Ga As-Ge or Ga Al As - GaAs are known.